The present invention relates to the field of turbine engines and to regulating them.
The term “turbine engine” is used in the present context to designate any machine for converting heat energy from a working fluid into mechanical energy by expanding said working fluid in a turbine. More particularly, the working fluid may be a combustion gas resulting from a combustion chemical reaction with air in a combustion chamber, after the air has been compressed in a compressor that is driven by the turbine via a first rotary shaft. Thus, turbine engines, as understood in the present context, comprise bypass or non-bypass turbojets, turboprops, turboshaft engines, and gas turbines, amongst others. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the working fluid through such a turbine engine.
In particular, the description relates to methods of regulating turbine engines that comprise at least a compressor, a combustion chamber downstream from the compressor, first and second turbines downstream from the combustion chamber, a first rotary shaft constrained to rotate at least with said compressor and said first turbine, a second rotary shaft constrained to rotate with the second turbine, the second rotary shaft nevertheless being free to rotate relative to the first rotary shaft, and a regulator for controlling the feed of fuel to the combustion chamber. Such turbine engines are known as “gas turbines” in particular for static applications, as “turboprops” when the second rotary shaft is used to drive a propulsive propeller, or as “turboshaft engines” when mounted on board a vehicle but used to drive a propulsive device other than a propulsive propeller. Thus, turboshaft engines are used in particular for driving the rotary wings of aircraft.
In this field, and more particularly for aircraft turboshaft engines and turboprops, and in particular for turboshaft engines that are to drive rotary wings, an accidental break in the power transmission from the second turbine, or “free” turbine, can lead to the second turbine running away dangerously. In order to avoid more severe damage as a result of such runaway, proposals have been made for the regulator to cut off the feed of fuel to the combustion chamber if the speed of rotation of said second rotary shaft exceeds a maximum threshold.
Setting this maximum threshold nevertheless requires major technical compromises. In certain applications, it is desirable to reach significant overspeeds for the second shaft during certain operating conditions of the engine. Thus, in the field of rotary wing aircraft, this can serve to allow transient peaks in the speed of rotation of the rotary wing in the event of the aircraft performing severe maneuvers. Nevertheless, a high maximum threshold for the speed of rotation of the second rotary shaft also requires significant overdimensioning of the second turbine and of the second rotary shaft compared with the maximum torque that is delivered by the second rotary shaft under stable conditions, in particular maximum continuous power (MCP) conditions or maximum takeoff power (TOP) conditions. Such overdimensioning can normally be achieved only to the detriment of the performance of the engine, and in association with a significant increase in its weight, where such an increase is particularly undesirable in the field of aviation.
Faced with this drawback in multiengine power plants having at least two such engines, one solution proposed in the state of the art is to incorporate a crossed inhibition device in the power plant serving to avoid simultaneously cutting off the feed to both engines as a result of said maximum speed threshold being exceeded by the second rotary shaft. Nevertheless, that solution presents other drawbacks, and in particular in the event of a break in the power transmission downstream from both engines during an accident.